This invention relates generally to a process and apparatus for the low pressure chemical vapor deposition of thin films, and more specifically to process and apparatus for the deposition of metallic films.
Low pressure chemical vapor deposition (LPCVD) is utilized for the formation of various thin films. Several of these films are essential in the fabrication of semiconductor devices including polycrystalline silicon, silicon oxide, and silicon nitride. More recently, interest has been generated in the LPCVD of refractory metallic films such as selective tungsten, tungsten silicide, titanium silicide, titanium nitride, and titanium boride. As applied to the electronics industry, these refractory metallic films are useful as improved gate electrodes and interconnections on MOS devices, contact enhancements to semiconductor materials, metallization of Schottky devices, diffusion barriers, and the like. In other industries the refractory metallic films are utilized to form hard protective coatings for tools, dies, and other types of machinery benefiting from such refractory surfaces.
The source gases for these binary metallic compounds nominally consist of a semivolatile metal source material such as WF.sub.6 or TiCl.sub.4 and the volatile hydride or halide of an inorganic source material such as SiH.sub.4, NH.sub.3, B.sub.2 H.sub.6, or BCl.sub.3 and H.sub.2. The deposition occurs through the metathetical reaction between the metal halide and inorganic hydride or halide to deposit a refractory metal film and liberate a hydrogen halide. For example, tungsten silicide and titanium boride are deposited in accordance with the following reactions. For the deposition of tungsten silicide EQU WF.sub.6 +2SiH.sub.4 .fwdarw.WSi.sub.2 +6HF +H.sub.2.
For the deposition of titanium boride, EQU TiCl.sub.4 +B.sub.2 H.sub.6 .fwdarw.TiB.sub.2 +4HCl+H.sub.2.
The binary metallic films formed by chemical vapor deposition and specifically by low pressure chemical vapor deposition tend to be uniform and to have very low concentrations of unwanted impurities. The stoichimetric uniformity results from the fact that the elemental composition of CVD films is strongly influenced by free energies of formation of compounds for specific stoichiometric values. The low concentration of unwanted impurities results from the fact that, during deposition, impurities such as oxygen, carbon, and other trace metals are conducted away from the deposition surface by volatile reaction by-products diffusing away from this surface. In addition, films deposited by LPCVD tend to be conformal and to yield excellent step coverage.
The deposition of some materials such as polycrystalline silicon is relatively straight forward. As described, for example, in U.S. Pat. No. 3,900,597, polycrystalline silicon is deposited on semiconductor wafers by loading multiple wafer boats into a tubular quartz reactor, evacuating the reactor tube, heating the tube to the desired temperature, and injecting source gases to obtain a uniform deposit across each wafer and from wafer to wafer. The deposition of refractory metals and other materials which are deposited from high molecular weight source materials is not as simple, however, as the deposition of polycrystalline silicon. This simplistic approach is not adequate to obtain uniform film thickness from wafer to wafer. The major film uniformity problems arise as a result of two factors: (1) The generation of reaction byproducts down the boat; and (2) The retardation of the diffusion rate of the high molecular weight source material as compared to lower molecular weight materials.
The adverse effect which arises from the generation of reaction by-products down the boat is primarily due to the build up of the respective hydrogen halide (HF and HCl in the two reactions described above). As the hydrogen halide concentration increases, the rate of a competing side reaction is also enhanced. For example, as tungsten silicide is deposited by the reaction EQU WF.sub.6 +2SiH.sub.4 .fwdarw.WSi.sub.2 +6HF+H.sub.2
The tungsten silicide is being depleted by the reaction EQU WSi.sub.2 +10HF .fwdarw.WF.sub.6 +2H.sub.2 SiF.sub.2 +3H.sub.2.
The build up of hydrogen halide and the resulting problem of nonuniformity is a generic problem which accompanies the deposition of metallic films which utilize metallic halide source materials.
At the reduced pressures used in low pressure chemical vapor depostition (generally less than about 65 Pa) the mean free path between molecular collisions approximates the geometries found in a typical LPCVD reactor. The diffusion rate of gases thus plays a primary role in their transport; and, therefore, the rate of their transport is approximately proportional to the inverse square root of their molecular masses. For example, at the reduced pressures of interest, SiH.sub.4 diffuses about three times as fast as WF.sub.6. Lighter by-product gases such as HF and H.sub.2 diffuse at even higher rates. The result of this disparity in diffusion rates is a non-uniform distribution of reactants along the reaction chamber; the high molecular weight reactant builds up in concentration near the input end in contrast to locations further along the reaction chamber.
To compensate for the build up and for the generally nonuniform distribution of reactants, it is essential to increase the concentration of the high molecular weight metallic source material down the reaction chamber. In an attempt to provide a uniform distribution of reactants, the prior art discloses an injection tube or manifold with holes spaced to allow the injection of gases along the entire length of the injection tube. This modification, however, does not yield uniform metallic films because most of the high molecular weight gas is ejected from the first few holes in the injection tube and virtually none is ejected from holes further along the tube. This adverse phenomenon results from the diffusion of lighter molecular weight source gases and reaction by-products back into the injector tube holes. The concentration of the lower molecular weight source gases, in turn, retards the diffusion of the higher molecular weight metallic source gases down the injector tube.
To further attempt to provide a uniform distribution of reactants throughout the reaction chamber, attempts have been made to inject the high molecular weight metallic source gases into the center of the reactor and to exhaust by-products from both ends of the reactor. This technique enhances the uniformity of the deposition, but requires substantially increased reactor complexity and complicates the loading, unloading and general handling of the deposition surfaces.
In considering the problem of achieving uniformity of deposition in a low pressure chemical vapor deposition process, a further problem must also be given consideration. High quality LPCVD films require precise control of the reaction ambient. Any leak in the deposition system which allows even minute amounts of air or other contaminant into the reaction system results in inferior films and can even lead to potentially dangerous reactions between the air and certain of the reactants. Accordingly, the reaction apparatus must be constructed so that the deposition surfaces can be placed in the chamber and subsequently removed without compromising the integrity of the sealed system. In addition, it is preferable that most, if not all, of the plumbing for vacuum lines, reactant and reactant by-product lines, and the like be designed to ensure leak-free operation of the system. It is especially important, for example, that the seal between the reactor tube and the end caps which enclose the ends of the tube be undisturbed once the system is placed in operation. This includes leaving the seals undisturbed when the composition or type of film deposited in the reactor is changed.
Although some of the foregoing problems can be overcome by using a refractory metal LPCVD reactor which deposits films upon a single deposition surface, these single surface systems have the disadvantage of low throughput, high unit cost, and increased complexity. Accordingly, a need existed for an apparatus and process for depositing thin films which provide uniform and high integrity films with a high throughput.
It is therefore an object of this invention to provide an improved process for the low pressure chemical vapor deposition of uniform films.
It is another object of this invention to provide improved apparatus for low pressure chemical vapor deposition.
It is still another object of this invention to provide an improved injector for injecting high molecular weight reactants in a low pressure chemical vapor deposition system.
It is yet another object of this invention to provide an improved process for the low pressure chemical vapor deposition of metallic films from high molecular weight metallic compounds onto semiconductor wafers.
It is still another object of this invention to provide an improved end cap and removeable feedthrough for a chemical vapor deposition reactor system.